Recently, wavelengths expansion (upgrade) in optical transmission systems such as existing submarine cables is performed as a means for increasing the transmission amount to respond to increase in communications amount. In a case where the number of wavelengths is increased in, for example, one-wavelength optical transmission system or a wavelength division multiplexing (WDM) system with a narrowband, wavelengths can be multiplexed in a high density with the wavelength intervals (grids) set to be narrow in a predetermined signal band. Further, error correction schemes and encoding schemes with improved characteristics can be demanded.
On an occasion of transmission of signal light through an optical transmission path under a poor optical signal to noise ratio (OSNR) state, significant effects from the polarization of signal light are caused, and changes in the signal light level and characteristics deterioration can be caused by polarization hole burning or the like. To reduce such effects as deterioration of signal quality due to the polarization dependency, a polarization scrambler that randomizes the polarization state of signal light can be used.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2003-60576